Video Card Image Quality Comparison

Reiterating our ealier comments, there has been a lot of noise lately about the “optimizations” to trilinear filtering present in both ATI and NVIDIA’s recent cards and drivers. In the GeForce FX series of cards, NVIDIA’s drivers forced on trilinear optimizations that sometimes used a full eight texture samples, but on some pixels used less, where it was determined that the full texture sampling required for typical trilinear filtering was not necessary. Some viewed this as a combination of bilinear and trilinear filtering and dubbed it “brilinear.” The point was to attempt to make the video card do less work while still achieving trilinear filtering’s goal of eliminating the visible transitions between mipmap levels.

If you think about it, this type of optimization is really a clever hack. We’re unsure of the exact algorithms used by NVIDIA and ATI  they won’t share that intellectual property  but the idea is to use trilinear filtering only where it makes the most difference, such as near the boundaries between mipmap levels. Perhaps it is never used on surfaces that are very distant from the camera. The trick is in how you determine where to make the shift from trilinear to bilinear.

ATI did the same thing in their Radeon 9600 line of cards, though nobody really seemed to notice, and it is present in the new X800 series as well. Our latest NVIDIA drivers have a switch to toggle this on and off on 6000-series cards, buried at the bottom of the “Performance and Quality” settings. This feature is still missing on GeForce FX cards. ATI’s drivers provide no such toggle; trilinear optimizations are always enabled by default.

Though there has been quite an uproar in the graphics-card watching community over these optimizations, do they really matter? Is image quality negatively impacted by the use of trilinear optimizations? Let’s take a look at a few scenes to see if any noticeable differences exist.

We’ll use the above scene from the game Painkiller throughout this article to judge image quality.

In this close-up of the floor at 4X magnification, we can see what the filtering looks like on the GeForce 6800 Ultra (with and without trilinear optimizations) and on the Radeon X800 XT. There are two important things to note. First is that there is virtually no visible difference between enabling and disabling the trilinear filtering optimizations on the 6800. Second, ATI’s texture filtering is noticeably better. We’ve circled in blue a couple of areas where the transitions between mipmap levels are apparent on the 6800, while the X800 more smoothly blends between levels, despite the fact that its own trilinear optimizations are locked on. But appreciate that we’re at 4X magnification here, and while the differences between nVidia and ATI are appreciable, they’re less so at normal magnification, especially when you’re ripping through the game blowing up zombies.

Here’s another scene we’ll use often, from Unreal Tournament 2003. The stone textures along the floor are good for examining filtering, while the rafters and walls will help us judge anti-aliasing quality when we get to that.

Under 4X magnification (and with the brightness cranked up), we still can’t really see any substantive difference in filtering quality here. The GeForce 6800 series cards look the same with and without trilinear optimizations, and the Radeon X800 cards don’t really look any different. We can’t see visible boundaries between mipmap levels.

We’ve examined quite a few other applications and the story is basically the same as you would get from our two cases presented here. First, enabling or disabling the trilinear optimization has no real visible impact on visual quality on the GeForce 6800 cards. The difference is certainly not something you can see with the naked eye, even looking at a still screenshot under magnification. Second, the X800 generally has identical trilinear filtering quality, but in a few cases creates a smoother transition between mipmap levels than the 6800.

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